1. Field of the Invention
The present invention relates to a portable, modular VOG system that provides for the calculation and display of saccade eye movement for objective analysis.
2. Background Information
The present invention relates to Video-oculography, also called VOG, and video-oculographic recording of eye movement has been shown to be a highly effective non-invasive technology for evaluating eye movement. See the Richard E. Gans article in the May 2001, volume 54, pages 40-42 of The Hearing Journal, which provide great insight to the beginning of practical goggle based VOG systems in 2001. As a historical note, for over 50 years, clinicians and researchers have depended upon electronystagmography (ENG or EOG) to evaluate peripheral and central vestibular function. Although ENG/EOG remains in significant use, the need to place electrodes around the eyes and the inability to record or observe a torsional nystagmus have represented significant limitations with this technology. Another drawback is that much of the testing must be performed with the patient's eyes closed. Therefore, clinicians and researchers have been dependent on the graphic, uni-dimensional recording to speculate as to what movement the eyes were actually making beneath closed eyelids.
Abnormalities of eye movement provide valuable information about the location of the dysfunction or disease process. Many abnormalities are specific to certain pathophysiology or pharmacologic influences. The advantage of recording/evaluating eye movements versus other axial or limb musculature is that they are easier to interpret. Eye movement is limited to movement in three planes: horizontal, vertical, and rotational. Pupil dilation represents another parameter that may be desired to be tracked for certain applications.
Eye movements may be categorized as those that stabilize vision during head movement and those that shift vision. The vestibular system stabilizes vision with head movement through the mechanoreceptors of the labyrinth, sensing the direction and speed of head acceleration and moving the eyes accordingly. When disease affects a particular semicircular canal within the labyrinth, nystagmus may occur in the plane of the involved canal. It is this anatomical and physiological relationship of the VOR that makes new technologic improvements in its assessment so important.
Current VOG systems that accurately track eye movement for diagnostic purposes can be represented by those described in U.S. Patent Application Publications 2005-0099601, 2007-0177103, 2007-0132841, 2008-0049186, and 2008-0049187 which are incorporated herein by reference. A further example of a current state of the art VOG system include the 2008 I-Portal® brand VOG systems from Neuro-Kinetics, Inc, which is a fully digital VOG system that delivers accurate 4D eye tracking. The lightweight goggle system is offered in standard 60 Hz and high-speed 120 Hz goggle sets, both occluded and free field of view. Higher speeds are available in customized applications with the integration of higher speed cameras, with the system currently accommodating up to 5000 hz cameras.
A saccade, for the purpose of this application, is a fast movement of an eye. Eye saccades are generally quick, simultaneous movements of both eyes in the same direction. Saccades serve as a mechanism for fixation, rapid eye movement, and the fast phase of optokinetic nystagmus. Humans, and other animals, do not look at a scene in a steady way. Instead, the eyes move around, locating interesting parts of the scene and building up a mental ‘map’ corresponding to the scene. One reason for saccades of the human eye is that the central part of the retina, the fovea, plays a critical role in resolving objects. By moving the eye so that small parts of a scene can be sensed with greater resolution, body resources can be used more efficiently.
In addition, the human eye is in a constant state of vibration, oscillating back and forth at a rate of about 30-70 Hz. These “micro-saccades” are tiny movements, roughly 20 arcseconds in excursion and are generally imperceptible. They serve to refresh the image being cast onto the rod cells and cone cells at the back of the eye. Without microsaccades, staring fixedly at something would cause the vision to cease after a few seconds since rods and cones only respond to a change in luminance.
Saccades are the fastest movements produced by the human body. The peak angular speed of the eye during a saccade reaches up to 1000°/sec in monkeys and somewhat less in humans. Saccades to an unexpected stimulus normally take about 200 milliseconds to initiate and then last from about 20 to 200 milliseconds, depending on their amplitude. The amplitude of a saccade is the angular distance that the eye travels during the movement. For amplitudes up to about 60 degrees, the velocity of a saccade linearly depends on the amplitude (the so called “saccadic main sequence”). For instance, an 10° amplitude is associated with a velocity of 300°/sec, and 30° is associated with 500°/sec. In saccades larger than 60 degrees, the peak velocity starts to plateau (non-linearly) toward the maximum velocity attainable by the eye.
Saccades may rotate the eyes horizontally or vertically, or in any oblique direction to change gaze direction (the direction of sight that corresponds to the fovea), but normally saccades do not rotate the eyes torsionally. Torsion can be defined as clockwise or counterclockwise rotation around the line of sight when the eye is at its central primary position. Head-fixed saccades can have amplitudes of up to 90° (from one edge of the oculomotor range to the other), but in normal conditions saccades are far smaller, and any shift of gaze larger than about 20° is accompanied by a head movement. During such gaze saccades, first the eye produces a saccade to get gaze on target, whereas the head follows more slowly and the vestibulo-ocular reflex causes the eyes to roll back in the head to keep gaze on the target.
There are many, some rare, abnormalities of eye movements that can be diagnosed through observation of saccades eye movements. See, for reference, Saccade Calibration Testing article by Dr. Timothy C. Hain at http://www.dizziness-and-balance.com/practice/saccade.htm, which discusses i: Disorders of Saccade velocity (Too slow and Too fast), ii) Disorders of latency (timing), iii) Disorders of Saccades Accuracy (Overshoot, Undershoot, Blindness, Glissades and Pulsion), and iv) Disorders with “Square Wave Jerks” (saccadic oscillations with a latency between each saccade). This background is believed to be known to one of ordinary skill in the art associated with the present claimed invention.
Microsaccades are a kind of fixational eye movement. They are small, jerk-like, involuntary eye movements, similar to miniature versions of voluntary saccades. They typically occur during prolonged visual fixation (of at least several seconds). Microsaccade amplitudes vary from 2 to 120 arcminutes. The role of microsaccades in visual perception has been a debated topic which, currently, is unresolved. It has been proposed that microsaccades correct displacements in eye position produced by drifts, although non-corrective microsaccades also occur. Microsaccades were also believed to prevent the retinal image from fading, but they do not occur often enough for that purpose, considering that perfectly stabilized images can disappear from perception in a few seconds or less. The current consensus is that all fixational eye movements are important for the maintenance of visibility.
The present application deals with secondary, and higher order, corrective saccades which may be considered as micro-saccades, namely corrective micro-saccades, following a main or primary saccade eye movement.
There remains a need in the art for a simple, and simple to use, VOG system effective for clinical and research applications.